Possible Audio Broadcast System

Hello,
I have been tasked with coming up with solutions for an audio broadcast system for use over the network. My biggest problem is finding an intrusive alert system so that all networked personnel will have the audio message simultaneously forced on them. We have a NT & 2000 based 100TX Ethernet network utilizing nortel routers. We don’t seem to have multicast capability yet. Can any one suggest an off the shelf solution or some relatively simple ways to develope the solution. I have already created a client application that responds to specific UDP commands but I want something that designed to be robust enough for an Emergancy Broadcast system.